Finish inverting the MC -> Object dependency.

There were still some disassembler bits in lib/MC, but their use of Object
was only visible in the includes they used, not in the symbols.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213808 91177308-0d34-0410-b5e6-96231b3b80d8